1. Field of the Invention
The present invention relates to a fuel injection control device which controls a quantity of the fuel injected from a fuel injection valve into an internal combustion engine by adjusting an opening period of the fuel injection valve.
2. Description of Related Art
A fuel injection control device is mounted on a vehicle to control the injected fuel, actually injected from a fuel injection valve into an internal combustion engine, at a required quantity determined based on operating states of the engine. To control the injected fuel at the required quantity, it is required to determine a fuel injection period of time (i.e., an opening period of the valve). However, an actual quantity of the injected fuel is changed with a pressure of fuel pumped up and sent to the valve. Therefore, it is required to detect the fuel pressure before the start time of the fuel injection and to determine the fuel injection period of time from the detected fuel pressure. Further, the fuel pressure is changed with time. Especially, after the fuel injection is started, the fuel pressure is considerably dropped. Therefore, it is desired to determine the fuel injection period from the fuel pressure detected just before the start time of the fuel injection.
Published Japanese Patent First Publication No. 2005-248721 discloses a fuel injection controller for determining a fuel injection period from a fuel pressure according to a first technique. In this technique, when the rotation of a crankshaft of the engine is started, a central processing unit (CPU) of a fuel injection control device performs an interrupt ion process to judge, based on a crank angle of the crankshaft, whether or not it is a timing to start a fuel injection. When it is now a fuel injection start timing, the CPU controls an analog-to-digital (A/D) converter to convert an analog fuel pressure signal, sent from a fuel pressure sensor, into a converted digital value (i.e., a detected fuel pressure value). Then, the CPU calculates a fuel injection period of time by using the converted digital value and a required quantity of the injected fuel, and the CPU determines both an on-timing and an off-timing of an injection pulse from the calculated fuel injection period and sets the on-timing and the off-timing in an injection pulse output timer.
The timer generates this injection pulse having a level change at cachet the on-timing and the off-timing, and a fuel injection valve is driven in response to the injection pulse. Therefore, the injection pulse acts as a driving signal. The injection pulse is also called an energizing pulse. When the on-timing set in the timer comes, the injection pulse set at an active level is sent from the timer to a driving circuit, and the driving circuit opens the fuel injection valve. In contrast, when the off-timing set in the timer comes, the injection pulse set at a non-active level is sent to the driving circuit, and the driving circuit closes the fuel injection valve.
Each of Published Japanese Patent First Publication No. 2002-303193 and the Publication No. 2005-248721 discloses a fuel injection controller for determining a fuel injection period from a fuel pressure according to a second technique. In this technique, when the on-timing of the injection pulse comes, the driving circuit starts the valve opening of the fuel injection valve, and the CPU starts the interruption process to control the A/D converter. Under this control, the A/D converter receives an analog fuel pressure signal from a fuel pressure sensor and converts the signal into a converted digital value. Then, the CPU calculates a fuel injection period of time by using the converted digital value and a required quantity of the injected fuel, and the CPU sets a time, elapsed from the on-timing by the calculated fuel injection period, in the timer as an off-timing of the injection pulse. The fuel injection valve is actually opened by the driving circuit at a time delayed from the on-timing of the injection pulse by a valve delay time.
However, in the first technique, to calculate the fuel injection period by using the fuel pressure, it is required to convert the analog signal indicating the fuel pressure into the converted digital value. Therefore, a fuel pressure required for the calculation of the fuel injection period is detected at a timing which is earlier than the on-timing of the injection pulse by plenty of time. This detected fuel pressure is sometimes considerably different from a fuel pressure at a timing just before the on-timing of the injection pulse.
Particularly, in case of a so-called multistage injection, a plurality of fuel injections are serially performed to repeatedly inject fuel from one fuel injection valve to the corresponding cylinder of the engine during one rotational movement of the cylinder. Therefore, at least one fuel injection is performed in a short fuel injection period. In this case, there is a high probability that an execution period of time required of the interruption process to set the on-timing and the off-timing in the timer for the present fuel injection overlaps with a fuel injection period of the preceding fuel injection. In this case, as a fuel pressure required for the calculation of a fuel injection period of the present fuel injection, a fuel pressure in the preceding fuel injection is sometimes detected. Therefore, in the first technique, the precision in the control of the fuel injection quantity is lowered.
Further, in the second technique, to calculate the fuel injection period by using the fuel pressure, the conversion of the analog fuel pressure signal into the converted digital value is performed in response to the on-timing of the injection pulse. Therefore, there is a probability that the fuel injection period is calculated by using a fuel pressure detected just before the start of the fuel injection. However, although the fuel injection period should be calculated based on a fuel pressure detected just before the start of the fuel injection, there is also another probability that the fuel injection period is calculated by using a fuel pressure detected just after the start of the fuel injection. The fuel pressure detected just after the start of the fuel injection is considerably lower than the fuel pressure detected just before the start of the fuel injection.
More specifically, the CPU performs various processes in order of priority. Therefore, even when the CPU receives a request for starting the interruption process, the CPU sometimes performs another process, having a priority higher than a priority of the interruption process, in a period of time including the on-timing of the injection pulse. In this case, because the start of the interrupt ion process is delayed from the on-timing of the injection pulse, a period of time from the on-timing of the injection pulse to the completion of the A/D conversion of the fuel pressure signal sometimes becomes longer than the valve delay time from the on-timing of the injection pulse to the actual valve opening. This means that the converted digital value obtained in this conversion is undesirably determined from a fuel pressure detected just after the start of the fuel injection.
Therefore, in the second technique, the fuel injection control device controls the fuel injection quantity with insufficient precision.